1. Field of the Invention
This invention relates to pulverizing and, more particularly, the use of an inert gas, such as carbon dioxide or nitrogen, to cool a feedstock of material, which is then pulverized.
2. Description of the Prior Art
Equipment which processes material by reducing its size and increasing its surface area is widely known in the art. Generally, various types of mechanical reduction methods are applied to materials to reduce the size of the material and increase its surface area as a function of weight or unit volume. There are four basic ways to reduce material: by impact, attrition, shear or compression. Apparatus known in the art employ one or a combination of these four methods to process material for subsequent use. Materials can be processed by impacting in one of two ways: by gravity impact or by dynamic impact. In gravity impact, the material is dropped onto a surface harder than the material being processed. This method is most often used to separate two materials which have different friability. The more friable material is broken while the balance remains intact. The friable material can then be separated from the remainder by screening. Gravity impact is frequently employed in the processing of coal by dropping it onto a hardened steel plate.
Dynamic impact typically involves a rotary hammer that accelerates the feedstock material towards breaker blocks or other hammers. Dynamic impact is used when a well-graded cubicle particle is the desired result of processing, to break ores along cleavage lines removing inclusions, and for materials too hard or abrasive for hammermills.
The attrition process reduces materials by subjecting them to forces between two hard surfaces. Typically, the material is first processed by the impact method using hammers and then further reduced by attrition of the material against screen bars. The process is appropriate for materials that are abrasive, such as limestone and coal.
In comparison, shear processing involves a trimming or cleaving action, as opposed to the rubbing action that occurs during attrition. Shear material processing is typically used during primary crushing when the desired reduction ratio is in the range of 6:1.
Compression processing is achieved by crushing material between two surfaces, one or both surfaces acting on the material being processed. Compression processing is used on extremely hard, tough, abrasive rock that breaks cubically. Compression material processing can be combined with attrition processing.
One or more of the crushing methods described above is used to process raw feedstock material into sizes more suitable for end use. Often, the methods are employed in series, each step reducing the size and increasing the surface area of the material being processed. For example, precious and semi-precious metal ores, like gold, silver and copper, are reduced in size and the surface area increased to facilitate the further refining of the ore into a finished product. Coal may be processed in a similar manner for use as a fuel source particularly in fluidized bed reactors or boilers. Generally, the combustion efficiency of a fluidized bed boiler using processed coal increases in proportion to the surface area of the coal fuel source. A third example of a finely processed material would be Portland cement, wherein finer particle sizes result in a stronger, more dense finished concrete composite product.
Processing material into finer and finer particulate sizes requires increasing energy consumption. Such processing also accelerates the wear on the equipment being used, particularly when the finished product is xe2x80x9cpulverizedxe2x80x9d for passing a xe2x88x92200 micron screen. The preprocessing of material feedstocks to facilitate a pulverized finished product, apart from using mechanical means, is unknown in the art.
An apparatus is provided for using a source of inert gas to significantly reduce the temperature of a feedstock material, increase its brittleness, and render the feedstock more susceptible to further processing, preferably pulverization. A method of using the apparatus is also described. The apparatus has a feedstock hopper which, by its dimension, controls the rate of material entering the apparatus. The feedstock is then exposed to a source of liquefied inert gas. As the feedstock material travels through the inlet, the liquefied inert gas expands and absorbs a tremendous amount of heat energy. The heat necessary to convert the liquefied gas to a gaseous state is absorbed from the feedstock material, cooling it significantly. In this cooled state, the feedstock material is extremely brittle and very little mechanical energy is required to pulverize it. The feedstock material then travels from the inlet into a crusher, such as a flywheel turbine. The blades of the flywheel turbine dynamically impact the feedstock material, reducing the material into powder. The housing of the flywheel turbine is adapted such that processed material is forced through an outlet once it has reached the proper particle size. A screen may be placed over the outlet to ensure uniform gradation of the finished product.
A wide range of materials can be processed using this invention. The pulverizer can be adapted to produce a wide variety of finished products by simply varying the dimensions of the inlet hopper, the inclination of the inlet tube, the rate of flow and type of inert gas used, and the dimensions and configuration of the crusher. Qualities of the finished product can be varied by regulating the flow rate of the inert gas, controlling cooling by using different inert gases, each having its own latent heat of evaporation, regulating the time to which the feedstock is exposed to the inert gas, and by varying the energy imparted by the crusher upon the material following cooling.